AN439 datasheet(4/16 Pages) STMICROELECTRONICS

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TRIAC turn-off description

AN439

4/16

Figure 4.

Relative variation of (dI/dt)c versus (dV/dt)c for a 4 A standard TRIAC

(typical values)

In practice, the current waveform, and thus the dI/dtOFF, is imposed by the load. Generally

we cannot change it.

So, in TRIAC applications, it is always necessary to know the dI/dtOFF of the load to choose

a TRIAC with a suitable (dI/dt)c. This is the most important parameter.

Suppose a circuit in which the dI/dtOFF reaches 2 times the specified (dI/dt)c. The standard

4 A TRIACs, characterized by the curves in Figure 4, will be not suitable even if the dV/dtOFF

is equal to 0.1 V/µs.

1.3

Application requirements

1.3.1

TRIAC with resistive load

In this case, the TRIAC current and the mains voltage are in phase (see Figure 5). When the

TRIAC switches off (i.e. when the current drops to zero), the mains voltage is equal to zero

at this instant and will increase across the TRIAC according to the sinusoidal law:

Equation 1

For the European mains, i.e. VRMS = 220 V at 50 Hz, the slope will be:

Equation 2

For 110 V, 60 Hz mains, the slope will be: dV/dtOFF ≈ 0.06 V/µs.

These relatively low dV/dtOFF correspond to the left points on the curves in Figure 4. The

dI/dtOFF only depends on the load rms current and the mains frequency. For resistive loads,

as for most other loads, we will have:

Equation 3

Area of spurious firing at

commutation

Safe area

Area of spurious firing at

commutation

Safe area

)

sin(

Max

Mains

)

(

)

(

RMS

)

(

OFF

≈

)

(

RMS

)

(

)

(

RMS

)

(

OFF

≈

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